an NPC cleric created a Lord's banner of Crusades with a hallow effect containing Death Ward as the spell in there. It makes those within a 40-foot radius and able to see the banner gain the benefits of the Death Ward spell.
a small army of npc monster hunters of around 180 NPC's, 20 NPC's staying behind for supporting the campsite outside. (ranging levels 3 - 10 )
A scary castle on top of a rock with a bunch of scary vampires inside.
The vampire lord master saluted them for their courage (from the safety of the castle tower) upon their arriving, and he opened up the castle gate at that after using an artifact to blot out the sun (temporarily) ; vampires don't die on exposure to sunlight in my campaign, but they do get a -8 penalty on all attributes while being exposed to it. (and if one drops below 0 they're rendered unconcious/helpless until the sun goes down again or if someone takes them out of the exposure)
Anyway, the plan was to use a makeshift battering ram and storm the gates after that. Now that the gates are opened, they're hesitant to storm in cause of course it will be an ambush of some sort.
Ways of how this may play out:
a) The party attacks with the main group and storms the gates. Past the gates, below the castles courtyard, there's a whole number of incorporeal undead (mostly wraiths) hiding in sewer tunnels/dungeon areas below the courtyard to rise up through the floor and attack the incoming army. The real ambush is that among them there are 3 banshees however strategically placed that will start wailing to maximum effect when the most of the army has gone through the gates. I do expect heavy losses here on the army side, but that's intentional, PC's ought to get the benefit of the banner and get a +4 on their saves if they choose to go this route.
b) Spamming greater invisibility and fly spells (they got a sorcerer among them) they can scale the walls of the castle on their own away from the main group. The ambush will still happen but they will be safe from the banshees as chaos envelops in the courtyard and find another way inside.
c) the castle has a small port on the seaside, they may try to sneak in through that way as well. (and be oblivious to what happens on the courtyard).
d) Whatever else they might think up I haven't taken into consideration yet.
After the Banshee ambush, the vampire lord will try to persuade remaining survivors to turn sides, promising immortality to those that will heed his call. (The monster hunter group has been infiltrated by a few of his minions of course)
The castle has numerous of steel pipes running through them, with little holes in them in each room. (they can be used for the vampires to assume gaseous form in and then quickly and safely make their way throughout the castle, or as an escape route to fall back and regroup, given they have fast healing)
Along their way to the BBEG, the vampire lord, they'll fight some gargoyles and his lieutenant minions
the BBEG vampire Lord, is a 20th level antipaladin, some of the higher level npc's will fight him in different skirmishes during the battlefield and fall to his blade... if the battle goes poorly he'll retreat to his last stand using gaseous form and the escape route steel pipes.
The last stand encounter when they go after him as he retreats into the blasphemous temple beneath the castle. He's heavily armored and will strike at them underwater in a flooded room with darkness active(doesn't need to breathe and has freedom of movement active).
Another thing planned is the holding cells/torture chamber where they keep their prisoners to feed from. All the poor souls that died down there have caused to form a Haunt, and will have the effect of a 'Harm' spell on any living creatures arriving in the room. (Saves apply of course). If anyone tries to dimension door or teleport within the castle walls, they'll get teleport trapped and redirected to this location.
The PC's got backup characters made that are also participating in the siege, and I warned them they might be needed. If they hesitate too long, or make poor decisions, their orders' army will suffer dearly for it. (the idea is that after the encounter the surviving PC's will lead the Order and rebuild them) ; so how much/many of the order survives depends on the PC's actions entirely.